1. Technical Field
This invention relates to cable television systems and, more particularly, to apparatus for transmitting data over a cable television channel susceptible to interference noise, the transmitted data being spread over at least a portion of the spectrum of the cable television channel.
2. Description of the Prior Art
The development of cable television systems has reached the stage where not only is the provision of two way information flow desirable but is practically required by the implementation of new services. For example, in the implementation of impulse pay-per-view service where the subscriber may impulsively select an event for viewing and assume a charge, at least one data channel is required in a direction from a cable television subscriber to a cable television headend to report service usage data. Other uses for a return path include power meter reading, alarm services, subscriber polling and voting and home shopping. While not every cable television system operator provides for two way transmission, manufacturers of cable television equipment have tended to provide for so-called upstream transmission in a direction from a subscriber toward a headend. Practically all such manufacturers provide so-called split or two way systems having a spectrum of frequencies for upstream transmission which at least includes a band from 0 and 30 megahertz. This band of interest comprises cable television channel T7 (5.75-11.75 megahertz), T8 (11.75-17.75 megahertz), T9 (17.75-23.75 megahertz) and T10 (23.75-29.75 megahertz). Whether a so-called "sub-split", "midsplit" or "high-split" system is applied for two way transmission by a headend operator, all three types of split transmission systems typically involve an upstream transmission in the 0-30 megahertz band of interest.
An article entitled "Two-Way Cable Plant Characteristics" by Richard Citta and Dennis Mutzbaugh published in the 1984 National Cable Television Association conference papers demonstrates the results of an examination of typical cable television (CATV) return plants. Five major characteristics in the 0-30 megahertz upstream band were analyzed. These include white noise and the funneling effect; ingress or unwanted external signals; common mode distortion resulting from defective distribution apparatus; impulse noise from power line interference and other influences; and amplifier non-linearities.
White noise and Gaussian noise are terms often used to describe random noise characteristics. White noise describes a uniform distribution of noise power versus frequency, i.e., a constant power spectral density in the band of interest, here, 0-30 megahertz. Components of random noise include thermal noise related to temperature, shot noise created by active devices, and 1/f or low frequency noise which decreases with increased frequency. The term noise floor is used to describe the constant power level of such white noise across the band of interest.
This noise is carried through each return distribution amplifier which adds its own noise and is bridged to the noise from all branches to a line to the headend. This addition of noise from each branch of a distribution tree in a direction toward a headend is known as noise funnelling or the funnelling effect. The constant noise floor power level defines a noise level which a data carrier power level should exceed.
The present invention is especially concerned with interference noise which causes peaks in the noise spectral density distribution in the band of interest. Interference noise destroys effective data transmission when known data transmission coding techniques such as frequency or phase shift keying are practiced. In particular, interference noise especially relates to four characteristics of return plant introduced above: ingress, common mode distortion, impulse noise and amplifier non-linearities.
Ingress is unwanted external signals entering the cable plant at weak points in the cable such as shield discontinuities, improper grounding and bonding of cable sheaths, and faulty connectors. At these weak points, radio frequency carriers may enter caused by broadcasts in, for example, the local AM band, citizen's band, ham operator band, or local or international shortwave band. Consequently, interference noise peaks at particular carrier frequencies may be seen in noise spectral density measurements taken on plant susceptible to ingress.
Common mode distortion is the result of nonlinearities in the cable plant caused by connector corrosion creating point contact diodes. The effect of these diodes in the return plant is that difference products of driving signals consistently appear as noise power peaks at multiples of 6 megahertz, i.e. 6, 12, 18, 24 and 30 megahertz in the band of interest.
Impulse noise is defined as noise consisting of impulses of high power level and short duration. Corona and gap impulse noise are created by power line discharge. Temperature and humidity are especially influential in determining the degree of corona noise, while gap noise is a direct result of a power system fault, for example, a bad or cracked insulator. The resultant impulse noise spectrum can extend into the tens of megahertz with a sin x/x distribution.
Amplifier nonlinearities or oscillations relate to pulse regenerative oscillations caused by marginally stable or improperly terminated amplifiers. The result is a comb of frequency peaks within the return plant band whose spacing is related to the distance between the mistermination and the amplifier.
All of these phenomena define interference noise as used in the specification and claims. The present invention provides an approximately 20 dB advantage in interference noise rejection over known modulated frequency or phase shift keying data transmission techniques.
From examining typical cable distribution plants, Citta et al. concluded that "holes" exist in valleys between peaks in the noise spectrum they plotted between 0 and 30 megahertz. They proposed that these valleys may be used to advantage by carefully choosing return carriers "slotted" in these valleys.
In follow-up articles published at the 1987 National Cable Television Conference, Citta et al. conclude that a 45 kilobit data signal may be transmitted by a coherent phase shift keying (CPSK) technique over carriers at 5.5 megahertz and 11.0 megahertz or in the vicinity of the T7 and T8 cable television channels respectively. While the choice of these carrier frequencies is claimed to avoid the noise distribution peaks caused by interference noise, there is considerable concern that such a modulated phase shift keyed data stream will run into noise peaks in a cable television distribution network outside of the investigations of Citta et al.
Other return path or upstream data transmission schemes have been tried. These schemes include, for example, the telephone system, described as "ubiguitous" by Citta et al. In other words, the return data path to a cable television headend is not provided over the cable television distribution plant at all. The serving cable is intentionally avoided either because of the interference noise problem in a split system or because the system is a one way downstream system. Instead, the subscriber's telephone line is used for data transmission. In this distance, however, there is concern that local telephone data tariffs may require the payment of line conditioning surcharges if the telephone line to a subscriber's home is used for data transmission in addition to normal "plain old" telephone service. Furthermore, the telephone line is only available when the subscriber is not using it, requiring an unscheduled or periodic data flow.
Another known return data transmission scheme involves the application of a separate data channel at a carrier frequency that avoids the troublesome 0-30 megahertz band. This scheme, of avoiding the noisy 0-30 megahertz band, is only possible in midsplit and high split systems.
So-called spread spectrum transmission of data is a technology which evolved for military requirements from the need to communicate with underwater submarines in a secure manner. Spread spectrum derives its name from spreading a data signal having a comparatively narrow bandwidth over a much larger spectrum than would be normally required for transmitting the narrow band data signal.
More recently the security advantages provided by spread spectrum transmission have been disregarded in favor of its capability of application in an environment of interference. For example, communications systems operating over a power line where impulse noise levels due to the power line are high have been attempted in the past but found to be only marginally acceptable, for example, power line plug-in intercom systems commercially available from Tandy Radio Shack. The Japanese N.E.C Home Electronics Group, however, has demonstrated a spread spectrum home bus operating at 9600 baud over an AC line in a home that is practical up to distances of 200 meters of power line. The NEC system has been characterized as the missing link between a coaxial cable (for example, a cable television cable) and an AC power line common to the majority of homes.
To understand spread spectrum and how it operates to eliminate the effects of interference, it is first important to define terminology surrounding pseudorandom noise or chip sequence generation. A binary pulse value of one or zero in a pseudorandom sequence is known as a chip. The speed of chip sequence generation is known as a chip rate. Rather than calling a sequence of bits a bit sequence, a pseudorandom sequence generated by a pseudorandom sequence generator is known as a chip sequence.
The process of spread spectrum involves the spreading of a comparatively narrowband binary data signal over a relatively broad frequency spectrum such as by mixing the binary data signal with a pseudorandom chip sequence at a much higher chip rate. The effectiveness of improving signal to interference ratio is related to the ratio of the bandwidth of the spread spectrum to the bandwidth of the data signal. The larger the ratio, the greater the effectiveness. When the pseudorandom chip sequence used to spread the data at the transmitter is correlated with an identical chip sequence generated at a receiver, the original data stream will be recovered. For broader band noise interference, the correlation process picks out the broadband wanted signal while keeping the interference broadband. For narrowband interference, the interfering signal is spread over the spread spectrum bandwidth. In both cases the correlation process provides a narrowband output for the wanted signal. The net effect of the entire process is a much enhanced signal-to-interference ratio.
Despite the development of the spread spectrum arts, the requirement remains in the cable television art for an upstream data transmission from a subscriber premises to a cable television headend that is impervious to interference noise.